A circuit is known from DE 10 2007 042 246 A1. In FIG. 1 of this document, a line-side converter 22 is connected to a machine-side inverter (20) via a DC link. The DC link has an energy storage unit 26 in the form of a capacitor. A so-called DC link voltage or intermediate circuit voltage is applied to the capacitor.
Particularly the machine-side inverter usually comprises switchable power semiconductor elements, for example IGBT's (IGBT=insulated gate bipolar transistor) or GTO thyristors (GTO=gate turn off), or the like. The power semiconductor elements can only be switched on and/or off within predetermined voltage limits. Switching into the non-conductive state outside of the voltage limits can result in damage to or even to destruction of the respective power semiconductor element. If, for example, IGBT's are present, the IGBT's can only be switched off, i.e. switched into a non-conductive state, if the intermediate circuit voltage at the capacitor does not exceed a predetermined maximum voltage.
It is however possible that, for any reason, the intermediate circuit voltage at the capacitor will increase and exceed the predetermined maximum voltage. In this case, DE 10 2007 042 246 A1 provides for a so-called chopper circuit 24 that is shown in FIG. 1 and that comprises a resistance and a transistor that is connected in series to the resistance. If the intermediate circuit voltage exceeds the predetermined maximum voltage, the transistor can be switched into a conductive state, resulting in a parallel circuit of the capacitor, to which the circuit voltage is applied, and the resistance. Consequently, the capacitor can discharge via the resistance and thus the intermediate circuit voltage decreases.
Apparently, DE 10 2007 042 246 A1 requires a higher expenditure, particularly with regard to the indicated chopper circuit.